1. Field of the Invention
The present invention relates to a display device for presenting a display by supplying a current video signal converted from a voltage video signal to a pixel circuit.
2. Description of the Related Art
Electroluminescence (hereinafter referred to as “EL”) display devices using a self-emissive EL element as an emissive element for each pixel offer advantages of, for example, self-emissive properties and reduced power consumption, and are therefore attracting attention for their potential to replace liquid crystal display devices (LCDs) and CRTs.
Active matrix type EL display devices wherein a switch element, such as a thin film transistor (TFT) for individually controlling EL elements, is provided for each pixel and the EL element is controlled for each pixel, can achieve especially high resolution displays.
In such an active matrix EL display device, generally, on a substrate, a plurality of gate lines extend in a row direction, a plurality of data lines and power source lines extend in a column direction, and each pixel is provided with an organic EL element, a selection TFT, a driving TFT, and a storage capacitor. By selecting the gate line, the selection TFT is turned on, a data voltage on the data line (voltage video signal) is charged in the storage capacitor, and this voltage turns on the driving TFT, thereby supplying power from the power source line to the organic EL element.
Japanese Patent Publication JP-A-2001-147659, (hereinafter referred to as “Document 1”) discloses a circuit in which two additional p-channel TFTs are provided for each pixel as controlling transistors and data current (current video signal) in accordance with display data is supplied to the data line.
In other words, Document 1 discloses a circuit in which the current video signal is supplied to the data line and to a current voltage conversion TFT to set a gate voltage of the driving TFT.
The circuit disclosed in Document 1 can set the gate voltage of the driving TFT in accordance with the data current flowing through the data line, so that the driving current of the EL element can be more accurately controlled compared with circuits wherein a voltage signal is supplied to the data line. Further, the number of elements can be relatively decreased by sharing the current voltage conversion TFTs.
In the configuration disclosed in Document 1, however, the data current must be supplied to the data line in order to drive each pixel circuit. Because video signals are ordinarily voltage signals, circuits, such as voltage-current conversion circuits for converting a voltage signal into a current signal, are required, and an IC (semiconductor integrate circuit) including the voltage-current conversion circuit must be additionally provided to supply the current signal to the display device from the IC.
Providing an additional IC including the voltage-current conversion circuit requires that the IC be separately prepared, entailing development and production costs, and therefore increasing the cost of the display device.
Although the voltage-current conversion circuit may be included in the display device to utilize the voltage-current conversion circuit used in the pixel of conventional active matrix EL display devices, improvement is required to overcome nonuniformity caused by variation in TFTs.